1. Field of the Invention
The present invention relates to an improvement on a continuously variable transmission which incorporates therein a toroidal continuously variable transmission for use as an automatic transmission for, for example, a vehicle (a motor vehicle), which realizes at low cost a construction which enables a quick and smooth mode switching between a low-speed mode and a high-speed mode.
2. Description of Related Art
The application of a toroidal continuously variable transmission to an automotive transmission is known as it is described in many publications such as Japanese Patent Examined Publication JP-B-2734583, Japanese Patent Unexamined Publication JP-A-5-39850, “supplement volume to Red Badge Series 245/A Book Teaches Latest Mechanisms of Motor Vehicles” pages 92 to 93 written by Motoo Aoyama and published by Sansuisha Co., Ltd./Kodansha Co., Ltd. on Dec. 20, 2001, and “Toroidal CVT” written by Hlirohisa Tanaka and published by Corona-sha Co., Ltd. on Jul. 13, 2000. Further, the application of the toroidal continuously variable transmission to the automotive transmission has already been implemented in some areas. In addition, a continuously variable transmission in which a toroidal continuously variable transmission is combined with a planetary gear transmission unit has also been known widely through publication in, for example, Japanese Patent Unexamined Publications JP-A-10-196759, JP-A-2003-307266, JP-A-2000-220719 JP-A-2004-225888 and JP-A-2004-211836. Among these documents, JP-A-10-196759 describes a continuously variable transmission including a mode (a low-speed mode) in which power is transmitted only by a toroidal continuously variable transmission and a mode (a high-speed mode) which realizes a so-called power split state in which main power is transmitted by a planetary gear transmission unit, which is a differential mechanism, while transmission ratios are regulated by the toroidal continuously variable transmission. In addition, the JP-A-2003-307266, JP-A-2000-220719, JP-A-2004-225888 and JP-A-2004-211836 describe continuously variable transmissions which include a mode (a low-speed mode) which can realize a so-called geared neutral state in which the rotational state of an output shaft can be switched between forward and reverse rotations across a halt state interposed therebetween while an input shaft is kept rotating in one direction.
FIGS. 9 to 10 show a continuously variable transmission described in the JP-A-2004-225888 and JP-A-2004-211836 which includes the mode which can realize the geared neutral state, in which FIG. 9 shows a block diagram of the continuously variable transmission and FIG. 10 shows a hydraulic pressure circuit which controls the continuously variable transmission. An output of an engine is inputted into an input shaft 2 via a damper 2. The power so transmitted to the input shaft 3 is then transmitted to a planetary gear transmission unit 5 which is a gear type differential mechanism directly or via a toroidal continuously variable transmission 4. Then, a differential component of constituent members of the planetary gear transmission unit 5 is taken out to an output shaft 9 via a clutch device 6, that is, low-speed and high-speed clutches 7, 8 shown in FIG. 10. In addition, the toroidal continuously variable transmission 4 includes input and output disks 10, 11, a plurality of power rollers 12, a plurality of trunnions (not shown) which each constitute a support member, an actuator 13 (FIG. 10), a pushing unit or loader 14, and a transmission ratio control unit 15. Among these constituent members, the input and output disks 10, 11 are disposed concentrically with each other and in such a manner as to rotate relative to each other.
Additionally, the power rollers 12 are held between inner surfaces of the input and output disks 10, 11 which face each other in such a manner as to transmit power (torque) between the input and output disks 10, 11. The trunnions rotatably support the power rollers 12, respectively. The actuator 13 is a hydraulic actuator and is adapted to displace the trunnions which support the power rollers 12, respectively, in axial directions of pivot shafts which are provided at both end portions of each of the trunnions so as to change transmission ratios between the input disk 10 and the output disk 11. In addition, the loader 14 is a hydraulic loading unit and is adapted to apply a load to the input disk 10 and the output disk 11 in a direction in which they approach each other. Additionally, the transmission ratio control unit 15 controls the displacement direction and amount of the actuator 13 so as to obtain a desired transmission ratio value between the input disk 10 and the output disk 11.
In the case of the illustrated example, the transmission ratio control unit 15 is made up of a control unit 16, a stepping motor 17 which is switched based on control signals from the control unit 16, a primary solenoid vale or line pressure control solenoid switching valve 18, a load solenoid vale or simply solenoid valve 19, a shift solenoid valve 20 and a control valve unit 21 whose operating state is switched by these constituent members 17 to 20. In addition, this control valve unit 21 is a combination of a transmission ratio control valve 22, a correction cylinder or correction cylinder 23, correction control valves 24a, 24b, high-speed clutch and low-speed clutch switching valves 25, 26 (FIG. 10). Among these constituent members, the transmission ratio control valve 22 is such as to control oil pressure that is supplied to or discharged from the actuator 13. In addition, the correction cylinder 23 is such as to regulate the loading state of the transmission ratio control valve 22 in response to a torque (a passage torque) which passes through the toroidal continuously variable transmission 4 so as to correct the transmission ratio of the toroidal continuously variable transmission 4. The correction control valves 24a, 24b are such as to control oil pressures that are supplied to or discharged from the correction cylinder 23 and are adapted to be switched in response to the switching of the solenoid valve 29. Furthermore, the high-speed clutch and low-speed clutch switching valves 25, 26 are such as to switch the introduction state of pressure oil to the high-speed clutch and low speed clutch 8, 7, respectively.
In addition, pressure oils taken out from oil pumps 27 (27a, 27b in FIG. 10), which are driven by power taken out from the region of the damper 2, are sent out into the control valve unit 21 and the loader 14. Namely, pressure oil sucked in from an oil reservoir 28 (FIG. 10) and discharged from the oil pumps 27a, 27b is regulated to become a predetermined pressure by a loader pressure regulator valve 29 and a low pressure side or secondary regulator valve 30 (FIG. 10). Of these valves, the loader pressure regulator valve 29 is regulated with respect to its valve opening pressure in response to introduction of an oil pressure based on a difference in pressure (a differential pressure) between a pair of hydraulic chambers 35a, 35b which are provided in the actuator 13 in such a manner as to face each other across a piston and an oil pressure based on the opening or closing of the line pressure control solenoid switching valve 18. Then, a pushing force or load generated by the loader 14 is restricted to an optimal value according to an operating condition of the vehicle based on the regulation of valve opening pressure in that way.
Additionally, the oil pressure that has been regulated by the loader pressure regulator valve 29 in that way is sent not only into the actuator 13 via the transmission ratio control valve 22 but also into an interior of a hydraulic chamber of the low-speed clutch 7 or high-speed clutch 8 via a manual oil pressure switching valve 31, a pressure reducing valve 32 and the low-speed clutch switching valve 26 or high-speed clutch switching valve 25. Of these low-speed clutch 7 and the high-speed clutch 8, the low-speed clutch 7 is engaged when realizing a low-speed mode for increasing a speed reducing ratio (including an infinite transmission ratio (geared neutral state=GN state)) and is disengaged when realizing a high-speed mode for reducing the speed reducing ratio. On the contrary, the high-speed clutch 8 is disengaged when realizing the low-speed mode and is engaged when realizing the high-speed mode. In addition, the supply and discharge of pressure oil to and from the low-speed and high-speed clutches 7, 8 is switched in response to the switching of the shift solenoid valve 20.
FIG. 11 depicts an example of relationship between a transmission ratio (a speed increasing ratio) of the toroidal continuously variable transmission 4 and a speed ratio (a speed increasing ratio) of the whole continuously variable transmission. For example, in the low-speed mode in which the low-speed clutch 7 is engaged, while the high-speed clutch 8 is disengaged, as is indicated by a solid line a, as the transmission ratio of the toroidal continuously variable transmission 4 is reduced from a value which can realize the GN state (a GN value), the speed ratio of the whole continuously variable transmission is made to be increased in a forward direction (+: a forward rotating direction) from a halt state (a state where the speed ratio is 0). In addition, as the transmission ratio of the toroidal continuously variable transmission 4 is increased from the GN value, the speed ratio of the whole continuously variable transmission is made to be increased in a reverse direction (−: a backward rotating direction) from the halt state. On the other hand, in the high-speed mode in which the high-speed clutch 8 is engaged, while the low-speed clutch 7 is disengaged, as is indicated by a solid line β, as the transmission ratio of the toroidal continuously variable transmission 4 is increased, the speed ratio of the whole continuously variable transmission is increased (in the forward direction).
In a vehicle which incorporates therein a continuously variable transmission like the one described above, an optimal speed ratio (a target speed ratio) of the continuously variable transmission is obtained by the control unit 16 based on a running state (an operating condition) of the vehicle at a certain point in time which is obtained from a manipulation of an accelerator pedal (a throttle position) or a running speed (a vehicle speed). Then, in order to realize this target speed ratio, the stepping motor 17 is driven based on a control signal from the control unit 16 so as to switch the transmission ratio control valve 22, so that the transmission ratio of the toroidal continuously variable transmission 4 is regulated to a target transmission ratio which corresponds to the target speed ratio. Additionally, in conjunction with this, the shift solenoid valve 20 is switched as is required (according to the target speed ratio of the continuously variable transmission) to thereby switch the engagement or disengagement state of the low-speed and high-speed clutches 7, 8, so as to select a required running mode (the low-speed mode or the high-speed mode), whereby the speed ratio of the continuously variable transmission is regulated to the optimal value (the target speed ratio) according to the running state then.
Incidentally, in the case of the continuously variable transmission which is made up of the toroidal continuously variable transmission and the planetary gear transmission unit which are combined together via the clutch device and which has the low-speed mode and the high-speed mode, whether it can realize the geared neutral state or the power split state as is described in the JP-A-10-196759, the mode switch between the low-speed mode and the high-speed mode will be performed in the following manner. Namely, this mode switch is performed in a state where the speed ratio of the continuously variable transmission that is regulated according to a running state at a certain point in time (or a target speed ratio which corresponds thereto) is regulated to a value which can be realized in both the low-speed mode and the high-speed mode (a value (of the order of 0.3 in a speed increasing ratio) which corresponds to an intersection point B between the solid line α denoting the low-speed mode and the solid line β denoting the high-speed mode in FIG. 11). As this occurs, when standing on the viewpoint from the toroidal continuously variable transmission 4, the mode switch is performed in a such a state that the transmission ratio of the toroidal continuously variable transmission 4 that is regulated according to a running state at a certain point in time (or a target speed ratio which corresponds thereto) is regulated to a mode switching point (a rotation synchronizing point, which is on the order of 0.4 in the speed increasing ratio (a maximum reduction state)) which is a value corresponding to the intersection point B.
For example, in the case of the vehicle running in the low-speed mode, when the transmission ratio of the toroidal continuously variable transmission 4 is reduced (the speed ratio of the continuously variable transmission is increased) according to a running state of the vehicle then to reach the mode switching point (for example, 0.4 in the speed increasing ratio), the shift solenoid valve 20 is switched based on a control signal of the control unit 16. Then, the high-speed clutch 8, which had been in disengagement until then, is engaged, while the low-speed clutch 7, which had been in engagement until then, is disengaged, whereby the running mode is switched from the low-speed mode to the high-speed mode. On the contrary, in the case of the vehicle running in the high-speed mode, when the transmission ratio of the toroidal continuously variable transmission 4 is reduced (the speed ratio of the continuously variable transmission is reduced) according to a running state of the vehicle then to reach the mode switching point, the shift solenoid valve 20 is switched based on a control signal of the control unit 16. Then, the low-speed clutch 7, which had been in disengagement until then, is engaged, while the high-speed clutch 8, which had been in engagement until then, is disengaged, whereby the running mode is switched from the high-speed mode to the low-speed mode.
When the mode switch is performed between the low-speed mode and the high-speed mode in the ways described above, it becomes important to perform the mode switch in a smooth fashion with a view to securing a good riding performance (riding comfort). As a technique for realizing such a smooth mode switch, for example, Japanese Patent Unexamined Publications JP-A-2002-139140 and JP-A-2002-276789 describe a technique in which in a continuously variable transmission which incorporates therein a toroidal continuously variable transmission (or a belt-type continuously variable transmission unit), a mode switch (reversal in a passage torque) is performed in such a state that the transmission ratio of the toroidal continuously variable transmission is held (fixed) to a predetermined value (in such a state that a change in speed is prohibited) so as to reduce a shock that is generated when the mode switch is performed. In addition, Japanese. Patent Unexamined Publication JP-A-11-108147 describes a technique in which a mode switch is performed in such a state that a rotational speed of a power that is inputted in to a planetary gear transmission unit by way of a toroidal continuously variable transmission coincides with a rotational speed of a power that is inputted into the planetary gear transmission unit without passing through the toroidal continuously variable transmission so as to enable the engagement of a clutch in a smooth fashion when the mode switch is performed. Additionally, JP-A-9-210191 describes a technique in which when a mode switch is performed, both of clutches one of which has been in engagement and the other in disengagement are brought into engagement simultaneously, whereafter the clutch that had been in engagement until then is disengaged so as to perform smoothly the engagement and disengagement of the clutches.
Then, Japanese Patent Unexamined Publication JP-A-2003-207042 describes a technique in which a timing when a low-speed clutch is disengaged when a running mode is switched from a low-speed mode to a high-speed mode is made to differ from a timing when a high-speed clutch is disengaged when the running mode is switched from the high-speed mode to the low-speed mode In addition, Japanese Patent Unexamined Publication JP-A-2003-194207 describes a technique in which the transmission ratio of a toroidal continuously variable transmission is corrected (regulated) in the midst of a mode switch to thereby reduce a gear change shock based on a torque shift. Additionally, Japanese Patent Unexamined Publication JP-A-2004-116576 describes a technique in which a gear change shock based on a torque shift which occurs when a mode switch (engagement and disengagement of clutches) is performed is reduced by regulating a timing to perform the mode switch in expectation of the torque shift.
Incidentally, as has been described before, the mode switch is preferably performed in such a state that the transmission ratio of the toroidal continuously variable transmission that is regulated according to the running state (or the target transmission ration which corresponds thereto) of the vehicle then is regulated to the mode switching point (the rotation synchronizing point). Because of this, according to the technique described in, for example, Patent Document No. 10, the regulation of the transmission ratio of the toroidal continuously variable transmission to the mode switching point is determined by a substantial coincidence of the rotational speeds of a fastening member and a fastened member of a clutch (a low-speed clutch or a high-speed clutch) that had been in disengagement until then. Then, it being determined from the substantial coincidence of the rotational speeds of the fastening member and the fastened member that the transmission ratio of the toroidal continuously variable transmission, the clutch, which had been in disengagement until then, is started to be engaged, while a clutch which had been in engagement until then is disengaged.
However, in the case that the clutch which had been in disengagement until then is started to be engaged on condition that the transmission ratio of the toroidal continuously variable transmission has reached the mode switching point in the way described above, the time period that is taken to perform the mode switch becomes long. Namely, when the modes are switched, the low-speed and high-speed clutches which make up the clutch device take a certain time period (cause a delay in responding to oil pressure) from the start to the end of engagement thereof in accordance with conditions at the time of switching such as oil temperature and temperature characteristics of friction materials which make up each of the clutches and furthermore based on an unavoidable delay in mechanical response. On the other hand, when the modes are switched, a change-speed control according to the mode which results after the intended mode switch is completed cannot be started before the engagement of the clutch which had been in disengagement until then is completed. Because of this, in the event that the engagement of the clutch which had been in disengagement until then is started on condition that the transmission ratio of the toroidal continuously variable transmission has reached the mode switching point, there occurs a case where the transmission ratio of the toroidal continuously variable transmission is held at the mode switching point for at least a time period from the start to the end of engagement thereof.
The longer the time period during which the transmission ratio of the toroidal continuously variable transmission is held unchanged in the way described above, in other words, the time period during which there occurs no change in speed of the toroidal continuously variable transmission becomes, the longer the time period which is taken before the change-speed control based on the mode resulting after the intended mode switch is completed becomes, and a time period which is substantially required for the mode switch is extended (the start of the change-speed control based on the mode resulting after the intended mode switch is completed is delayed). Then, when the time period required for the mode switch is extended, there might be caused a possibility that an acceleration intended by the driver cannot be obtained sufficiently (or the acceleration performance is reduced) when the vehicle is attempted to be started drastically from rest (or accelerated drastically), for example, by largely depressing the accelerator pedal (by increasing the throttle position) or when the vehicle is attempted to accelerated drastically based on a kickdown operation during the running of the vehicle, which is not preferable.